Electric field dependence of poling response for nematic liquid crystalline main chain polymers with large second order optical nonlinearities

We report on the characterisation of the second order nonlinear optical pro perties of a semiflexible main chain random copolymer with a head to tail s tructure synthesised from 4-carboxy-4'-[N-(11-hydroxyundecyl)-N-methylamino ]-azobenzene and hydroxybenzoic acid. This polymer exhibits a nematic liqui d crystal mesophase with a nematic-isotropic transition temperature of 200 degreesC. We find a complicated phase behaviour for the polymer and show th at the polymer films be readily annealed into large homogeneously aligned m onodomains in device configurations. We have carried out measurements of th e field dependent poling behaviour by means of an ellipsometric set-up afte r Teng and Man. We find three distinctive field regions corresponding to di fferent mechanisms in the response of the polymer chains. No saturation in the poling response can be detected for fields up to 10(8) V/m. Absolute va lues for chi ((2)) reach 80 pm/V. We analysed any potential chi ((3)) contr ibution to the total NLO signal and showed that this contribution can be ne glected for our measurements. (C) 2000 Elsevier Science S.A. All rights res erved.